Until now, a battery system which includes a rechargeable secondary battery and supplies electric power to a device mounted on a vehicle or the like has been known. In such a battery system, relays (contactors, electric relays) are provided on each of a positive terminal side and a negative terminal side of the secondary battery in order to switch the conduction states between the secondary battery and the device.
PTL 1 discloses a power supply device for detecting melting of a contactor provided on each of the positive terminal side and the negative terminal side of a battery for vehicle traveling. This power supply device has a simple voltage detection circuit that measures voltage on each of the battery side and the output side when the contactor is turned off by using a switch connected to this simple voltage detection circuit. As a result, when the voltage on the battery side equals to the voltage on the output side, it is determined that the contactor is melted to adhere.